Added fsync on/off support.
fs: sync: enable fsync by default.

Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
(cherry picked from commit 255c87e6afafed3d7b6a03add8eb409a5f34812b)
fs: sync: extend Franco's fsync disable

- Change name of variable from "fsync_enabled" to "fsync_disabled"
- Change variable location to sysctl "fs.fsync_disabled"
- Change fsync_disabled semantics:
	0 - fsync is enabled (default)
	1 - fsync is disabled
	2 - fsync is only enabled for datasync
- Enable fsync attempted/allowed stats